1. Field of the Invention
The present invention relates to a tubular extrusion die and, more particularly, to a single lip rotary tubular extrusion die for making a multi-layer co-extruded plastic film.
2. Discussion of the Prior Art
It is known that a tubular extruded thermoplastic film has a high degree of molecular orientation in the machine direction (MD) and a relatively low degree of molecular orientation in the transverse direction (TD) (sometimes referred to as the cross direction (CD)), causing an extruded thermoplastic film to have a low mechanical strength in the transverse direction. It is also known that a tubular extruded thermoplastic film can be stretched, as by blown expansion, to improve the molecular orientation, structural integrity and strength properties in the transverse direction. However, the amount of transverse direction orientation which can be imparted to an extruded film by blown expansion is limited by the degree to which the film can be stretched without breaking. In general, it would be desirable to increase the amount of transverse direction orientation of a thermoplastic film prior to expansion to increase its strength in that direction.
Attempts have been made to rotate a tubular die during extrusion to partially orient to molecular chains in a transverse direction to help offset the general machine direction orientation of the molecules which normally occur during extrusion. One such attempt in illustrated in U.S. Pat. No. 4,358,330. However, it is generally difficult to rotate an entire die requiring a complex and expensive die supporting structure. Moreover, since orientation inherently takes place outside of the die, there is a limit to how much transverse direction orientation can be imparted to the film because of the location of the frost line. Furthermore, too much twisting of the plastic melt will cause collapse of the tubular film, making this method of obtaining transverse direction orientation difficult to properly implement.
Another problem typically associated with tubular extrusion dies is the attainment of a uniform distribution of melt circumferentially of the die which is required if a uniform extrusion of melt in the thermoplastic film is desired. Prior art dies require the use of high internal pressure drops to circumferentially distribute the melt about the die which in turn requires very close internal tolerances in the flow paths, making machining tolerances very critical and further adding to the cost and complexity of the die. These high pressure drops also limit the output of material through a die.
It is also frequently desired to impart particular structural properties to an extruded film by layering melts of the same or different thermoplastic materials in the extruded film. This too requires a complex die construction which must be machined to a high tolerance to ensure a uniform layering of the various melts in the extruded film.
Prior art extrusion dies are also typically constructed to produce a single type of extruded film product from a particularly thermoplastic material. For extrusion of different types of films, using different units, different dies are required, which adds to the expense of an extrusion plant.
Another problem particularly associated with dies which contain a rotary part is that they must be sealed at various locations between rotating and non-rotating parts to prevent the escape of a melt during extrusion. Sealing is particularly difficult and expensive as high extrusion pressures are developed.